The present invention relates to a combustion chamber for fluid-bed combustion and having side walls and a bottom wall with orifices in the latter through which fluidizing and combustion air is injected into the combustion chamber.
It is well-known that for effecting fluid-bed combustion a fluidized bed or layer of an inert material, such as sand, ash, limestone or dolomite, in which the combustion takes place, is maintained in the lower part of the combustion chamber. Among the important advantages of this technique may be mentioned the lower combustion temperature as compared to combustion directly within a surrounding body of air, whereby the content of certain noxious substances, such as oxides of nitrogen, in the flue gases, is reduced. By choosing an inert material, which reacts chemically with sulphur, it is possible to burn solid or liquid fuels having a high sulphur content with minimum emission of sulphur compounds whereby costly equipment for desulphurizing the flue gases are not required.
It is known to inject the air which serves for keeping the inert bed material in the fluidized condition and for maintaining the combustion, through orifices formed in a bottom wall made of ceramic material. It is also known to inject the air through nozzles projecting from a bottom wall made of steel such that during operation of the combustion chamber there is formed a stagnant layer of inert material between the bottom wall and the level of the nozzle orifices. Since no combustion takes place in that layer, it acts as a thermal insulation of the bottom wall which thus in operation can be kept at a temperature below that prevailing in the fluidized bed.
In both these known structures the bottom wall may, however, in certain circumstances, in particular after interruption of the combustion process, be in danger of damage due to excessive heating from the glowing material above the wall. It has also been ascertained that the temperature distribution across the area of the bottom wall can be very uneven in the known structures which may result in warping of the bottom wal in response to non-uniform thermal expansion.